spec: fix mysql failures in quiz statistics due to no sub-second precision

refs CNVS-7032

also, don't load all quiz submissions just to find the latest timestamp

Change-Id: Ibcd2804644300aa861fbefba0917bd49b97aeb75
Reviewed-on: https://gerrit.instructure.com/28640
Reviewed-by: Anthus Williams <awilliams@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>